Collagen fibril characteristics at the corneo‐scleral boundary and rabbit corneal stromal swelling

Averaged spacing and 2-d organization of collagen fibrils in the posterior cornea of the rabbit eye assessed by transmission electron microscopy

PURPOSE

To assess how reproducible collagen fibril spacing might be in the corneal stroma as viewed by transmission electron microscopy by calculating averaged values for the 2-D organization.

METHODS

One cornea from 8 albino rabbits (2 kg) was fixed in situ to preserve natural shape. Thin sections were stained with 2% phosphotungstic acid (PTA) and images taken of fibrils from the central-posterior stroma. After projection at 250,000 X magnification, an overlay was prepared of the fibrils. Using a 500 × 500 nm region of interest (ROI), the distances to all fibril centers were measured to 2 nm resolution.

RESULTS

The sets of fibrils had average diameters between 32.4 and 36.1 nm (group mean ± SD of 34.4 ± 4.2 nm). The mean fibril density was 396 ± 21 per square micrometer, with a fibril area fraction of 38.7 ± 3.9%. The mean distance to the literal nearest neighbor fibril center was 43.2 ± 4.5 nm. A radial distribution analysis showed a distinct nearest neighbors peak at 51 nm. This nearest neighbors peak had an average amplitude of 2.236 ± 0.315, with a broader secondary peak being evident in all data sets centered at 93 nm with an average amplitude of 1.166 ± 0.093 (or 53.3 ± 7.1% of the nearest neighbors peak).

CONCLUSIONS

Overall, these results show that a predictable 2-D organization can be demonstrated for collagen fibrils in rabbit corneas when consideration is given to sample selection and preparation and the image analysis strategy.

Ultrastructure alteration in the corneal stroma of hydrated camel corneoscleral button and corneal button

PURPOSE

To investigate the changes in the ultrastructural architecture of hydrated corneoscleral button (HCSB) and hydrated corneal button (HCB).

METHOD

Corneoscleral buttons (n = 4) and corneal buttons (n = 4) were hydrated in deionized water for 24 and 48 h and were fixed in 2.5% glutaraldehyde containing cuprolinic blue in sodium acetate buffer and processed for electron microscopy. The tissue was dehydrated and embedded in TAAB 031 resin. Normal CSB and CB were also processed using the same method. Ultrathin sections were stained with uranyl acetate and lead citrate. The sections were observed under JEOL 1400 transmission electron microscope.

RESULTS

In both HCSB and HCB, the lamellae were disorganized and proteoglycans (PGs) were degenerated and detached from the collagen fibrils (CFs). In the HCSB samples, interfibrillar spacing increased but the diameter of the CF had not changed compared with those in the normal tissue. Within the HCSB, the interfibrillar spacing in the posterior stroma was significantly higher compared with those in the anterior and middle stroma. In the HCB, both the interfibrillar spacing and CF diameter had significantly increased compared to those observed in both the HCSB and normal tissue. The CFs were severely damaged exhibiting very thin microfilaments within CFs. Remnants of the CFs were present in the stromal matrix.

CONCLUSION

Hydration of the cornea in both the HCSB and HCB increases the interfibrillar spacing in the anterior, middle, and posterior stroma. The CF in the HCB was enlarged and had degenerated. This could be due to penetration of water into the intermolecular spacing within the CF.

Assessment of collagen fibril spacing in relation to selected region of interest (ROI) on electron micrographs--application to the mammalian corneal stroma

AIMS

To evaluate measurements of collagen fibril spacing using different shaped regions of interest (ROI) on transmission electron micrograph (TEM) images of rabbit corneal stroma.

METHODS

Following glutaraldehyde fixation and phosphotungstic acid staining, TEM images of collagen fibrils in cross section were projected at a final magnification close to 250,000 × to obtain overlays. Interfibril distances (IFDs; center-to-center spacing) were measured within different ROIs of the same nominal area (0.25 μm(2) ) but different shape (with the length to width, L:W, ratio from 1:1 to 6:1). The IFD distribution was analyzed, and the 2D organization assessed using a radial distribution analysis.

RESULTS

The fibrils had an average diameter of 35.3 ± 3.8 (SD) nm, packing density of 393 ± 4 fibrils / μm(2) and a fibril volume fraction of 0.39 ± 0.02. IFDs ranged from 29 to 1400 nm depending on the shape of the ROI, with average values ranging from 263 to 443 nm. By artificially selecting IFD data only to a radial distance of 250 nm, the average IFDs were just 145-157 nm. The radial distributions, to 250 nm, all showed a nearest neighbors first peak which shifted slightly from predominantly at 45-54 nm with more rectangular ROIs. The radial distribution profiles could be shown to be statistically different if the ROI L:W ratio was 2:1 or greater.

CONCLUSION

Selection of an ROI for assessment of packing density and interfibril distances should be standardized for comparative assessments of TEMs of collagen fibrils.

Observations on the ultrastructure of equatorial scleral collagen fibrils in sheep eyes

OBJECTIVE

To investigate collagen fibrils of the equatorial sclera in relation to the age-related changes in eye size in sheep.

ANIMALS STUDIED

Lambs and outbred ewes.

PROCEDURES

Sheep eyes (three lamb and three from adult outbred ewes), presumed disease-free, were processed for transmission electron microscopy (TEM) immediately postmortem. Tissue blocks from the equatorial region were sectioned across fibril bundles orientated along the equator. Micrographs including at least 500 fibrils were projected at 22,000× magnification for measures of fibril diameters (FDs).

RESULTS

Lamb eyes were smaller than those of adult ewes but equatorial scleral thickness was only marginally less at 0.232±0.013 vs. 0.254±0.012 mm (P value not significant). Scleral tissue was composed of compacted bundles of collagen fibers that tended to be rounder in outer compared to being flatter in inner regions. In typical (normal) appearing regions, FDs were distinctly larger (68-410 nm) in outer sclera compared to inner sclera (63-281 nm). Outer sclera FDs were bimodal averaging 192±58 nm, compared to unimodal distributions at inner locations averaging 156±48 nm (P<0.001). Some atypical regions, especially at outer-mid sclera locations, were also noted where the FD distribution was bimodal but also included numerous microfibrils (<50 nm diameter), with similar appearances being found for both lamb and adult ewe eyes.

CONCLUSIONS

The equatorial sclera is a mixture of rounder versus flatter collagen fiber bundles, the former being more likely to be made up of a mixture of both smaller and larger fibrils, as compared to slightly smaller fibrils.

Corneal responses to intraocular pressure elevations in keratoconus

PURPOSE

To examine corneal responses to elevated intraocular pressure (IOP).

METHODS

For a sample of 10 normal subjects, noncontact tonometry was used to measure IOP elevations in response to scleral indentation from a standardized ophthalmodynamometer (ODM) force. Using the same ODM force, corneal topography was assessed for the same controls and a sample of 10 subjects with keratoconus (KC). It was assumed that the mean and range of IOP elevations were similar for both samples.

RESULTS

The ODM induced a mean IOP elevation for the control eyes of 99.4%. IOP elevation during topography was 15-20 seconds for both samples. With elevated IOP, there were no significant topographical changes for control subjects, but the mean values for steepest point of curvature and flat and steep simulated keratometry were significantly increased in subjects with KC [+1.84 (P < 0.029), +0.64 (P = 0.046), and +1.31 diopters (D) (P = 0.03), respectively]. The changes were significantly greater in subjects less than 30 years (P < 0.05). There were no significant topography changes from baseline after IOP elevation, for either control or KC samples.

CONCLUSIONS

Abnormal elastic (reversible) increased distensibility in some KC corneas is consistent with reduced corneal rigidity (lower elastic modulus and/or thickness). Abnormal distending responses may be increased when IOP elevations are higher and/or longer and/or more frequent. The results suggest that abnormal distending responses to elevated IOP in KC may reduce with age.

Assessment of the apparent intra- and inter-sample variability in the collagen fibril diameter in the posterior corneal stroma of rabbits. A transmission electron microscopy study

PURPOSE

To assess the natural variability in the diameter of small collagen fibrils in the lamellae of corneal stroma of healthy young rabbits.

MATERIALS AND METHODS

The corneas of 6 young adult female New Zealand White rabbits (2.1 kg) were prepared for transmission electron microscopy (TEM) by fixation with a cacodylate-buffered 2% glutaraldehyde (pH 7.2-7.4, 320-340 mosm/kg). The corneas were embedded in Epon-Araldite, thin sections prepared from the central region of the posterior corneal stroma and stained with acidic uranyl acetate followed by acidic lead citrate. High magnification (x 33,000) micrographs were taken, and fibril diameters (FDs) measured at a final magnification of x 275,000 to a resolution of 2 nm.

RESULTS

Assessment of sampling-related errors indicated that the average diameter of the fibrils within any particular micrograph could be estimated to within 1% or better by measures of 100 fibrils. Assessments of the intra-sample variance (6 micrographs taken from the same cornea) indicated a group mean FD of 32.4 +/- 3.6 nm, whilst the inter-sample variance (6 micrographs taken from 6 different corneas) yielded an average of 33.1 +/- 4.5 nm (n = 100 fibrils/micrograph, +/-SD). However, group-averaged data sets of FDs, while unimodal, were not normally distributed, and cumulative averaging indicated a fixed range of FDs across the data sets.

CONCLUSIONS

Intra- and inter-sample variability in collagen FDs is very similar, but the analysis indicates that the collagen fibrils are not homogeneous and that closely adjacent lamellae can have subtle differences in average FD.

Assessment of the number of lamellae in the central region of the normal human corneal stroma at the resolution of the transmission electron microscope

PURPOSE

To determine the total number of lamellae within the central region of the human corneal stroma by using a continuous section through the corneal stroma and at the ultrastructural resolution of the electron microscope.

METHODS

Six human eye bank corneas, from individuals aged between 42 and 82 years, were received in corneal storage medium (Optisol GS) and then processed for transmission electron microscopy with buffered 2% glutaraldehyde fixation. Thin sections were mounted on parallel bar copper grids for an uninterrupted full corneal thickness observation. A sequence of overlapping micrographs were taken at 2,600x magnification, printed at a final magnification of 3,640x, and assembled as a montage. The number of lamellae were counted across the corneal stromal strip by three observers, independently, by using a preagreed set of criteria for identifying individual lamella.

RESULTS

The average number of lamellae per stroma was determined to be 242+/-4 (range, 234-247). In the more anterior region of the stroma, the density of the lamellae was 50% greater than in the posterior stroma (mean, 57+/-12 per 100 microm anteriorly vs. 38+/-5 per 100 microm posteriorly). Interobserver differences were modest and generally less than the intersample variance.

CONCLUSIONS

When assessed at the resolution of the transmission electron microscope, the number of stromal lamellae in the central region of the human cornea are higher (at approximately 240 per cornea) than most previous estimates reported from light microscopy studies.

Subjective vs. objective analysis of the corneal endothelial cells in the rabbit cornea by scanning electron microscopy - a comparison of two different methods of corneal fixation

OBJECTIVE

To illustrate a method of quantitative scanning electron microscopy (SEM) for the evaluation of the corneal endothelia of small animals by comparing two commonly used fixation methods.

ANIMALS STUDIED

Female New Zealand white rabbits, aged 10-12 weeks.

PROCEDURES

The corneas were either dissected from the eye and placed in fixative (2% glutaraldehyde in 80 mm cacodylate, pH 7.4) or the whole eyeball immersed in fixative solution and the cornea dissected later. Lower (x 200) and high magnification (x 1,000-5,000) images were inspected for overall appearance. Magnification images (x 500) were used to measure the areas of 100 cells, and cell density (CD) calculated from the average area.

RESULTS

Both fixation protocols yielded an intact endothelial surface, but the dissect- then-fix protocol resulted in more creasing and distortion artifacts that were avoided with the whole-eyeball fixation. Overall, the CD values were higher if the dissect-then-fix method was used, and the uniformity of the cell mosaic was less. The median CD values for the central, mid-peripheral and peripheral regions following dissect-then-fix protocols were 7,693, 7,353, and 7,071 cells/mm(2) (average eight corneas at each location). If the whole-globe fixation was used, the median CD values were 6,098, 5,747, and 4,785 cells/mm(2).

CONCLUSION

Cell density values in SEM can be very different according to the fixation method used. A distinct regional difference in CD was evident, which was more pronounced if the cornea was fixed prior to being dissected from the eye.

Biomechanically Coupled Curvature Transfer in Normal and Keratoconus Corneal Collagen

PURPOSE

To examine the hypotheses that a "pushed" biomechanically coupled intrafibrillar transfer of curvature may be involved in corneal warpage (pseudo cone) development, which is observed with superior cornea-bearing rigid contact lens fittings, and that a "pulled" biomechanically coupled intrafibrillar transfer of curvature may be involved in keratoconus cone development.

METHODS

The mechanical properties of corneal stromal fibrils have been reviewed to examine the histologic basis for fibril inextensibility and the associated potential for them to demonstrate biomechanical coupling.

DISCUSSION

The hypothesis that apical clearance rigid contact lenses may promote pushed biomechanical responses that aid cone development in some normal or keratoconic corneas is examined.

CONCLUSIONS

Stromal fibrils that run limbus to limbus and have circumferential stability at the limbus, appear to be relatively inextensible: a finding that is consistent with the need for stable refraction. The property of inextensibility could also support intrafibrillar biomechanically coupled curvature transfer. For example, in some normal corneas, definite apical clearance fittings may be associated with biomechanically coupled push forces generated by midperipheral bearing that squeeze the central cornea to greater curvature. A disease-related biomechanically coupled pull mechanism for cone formation may be accelerated by an apical clearance fitting push mechanism, with additional synergy generated by subatmospheric pressure in the postlens tear pool. These fitting-related mechanisms appear likely to increase with the degree of apical clearance. Cone formation in keratoconus and models of external forces acting on the cornea (tonometry and contact lens-related) are mathematically analyzed for an idealized one-dimensional elastic model of a corneal stromal fibril. The classic theory of "elastic rods" is adopted, and despite the nonlinear nature of the differential equations involved, a method of dealing with biomechanically coupled curvature transfer in an "exact" manner is described and used to model the related clinical phenomena.

Resolution and reproducibility of measures of the diameter of small collagen fibrils by transmission electron microscopy--application to the rabbit corneal stroma

AIM

To assess the impact of measurements of different numbers of small collagen fibrils at different final magnification values on the resultant mean values for fibril diameter (FD).

METHODS

Very high magnification (33,000x) electron micrographs were taken of the posterior-central zone of the corneal stroma from young adult rabbits (2 kg), printed at 46,000 or 50,000x, scanned at 400 d.p.i. and 35 mm slides prepared. These were projected to give final magnifications between 150,000x and 450,000x. An overlay of fibril outlines was prepared from the projected images and the fibril diameters (FD's) measured to within 0.5 mm.

RESULTS

On the overlays, at different projection magnifications, the average FD's ranged from 4 to 13.5 mm to allow measures to be made at a real resolution of between 3.5 and 1 nm. Using a fixed sized region of interest (ROI) on the overlays, the average values of FD's ranged between 24.9 and 31.7 nm, and could vary (for any particular micrograph) by up to 3.6 nm according to the number of FD's measured (n=34-384/ROI). Using a fixed number of FD measures (n=100) at different projection magnifications yielded average FD values from different corneas of between 25.1 and 35.2 nm, which could vary by up to 4.3 nm depending on the magnification used.

CONCLUSIONS

The results indicate that different average values for measures of fibril diameter of small collagen fibrils can easily be obtained according to the final magnification used and the number of fibrils measured, and that the overall averages can depend on whether the data sets are averaged or pooled. These aspects of the morphometry of small collagen fibrils therefore need to be carefully specified.